Fabian Stutzki

Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers

We report on the observation and experimental characterization of a threshold-like onset of mode instabilities, i.e. an apparently random relative power content change of different transverse modes, occurring in originally single-mode high-power fiber amplifiers. Although the physical origin of this effect is not yet fully understood, we discuss possible explanations. Accordingly, several solutions are proposed in this paper to raise the threshold of this effect.

Fiber chirped-pulse amplification system emitting 3.8 GW peak power

We report on the experimental demonstration of a fiber chirped- pulse amplification system capable of generating nearly transform-limited sub 500 fs pulses with 2.2 mJ pulse energy at 11 W average power. The resulting record peak power of 3.8 GW could be achieved by combining active phase shaping with an efficient reduction of the acquired nonlinear phase. Therefore, we used an Ytterbium-doped large-pitch fiber with a mode field diameter of 105 µm as the main amplifier.

High average power large-pitch fiber amplifier with robust single-mode operation.

Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 μm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.

Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers

The temporal behavior of mode instabilities in active large mode area fibers is experimentally investigated in detail. Thus, apart from the onset threshold of mode instabilities, the output beam is characterized using both high-speed camera measurements with 20,000 frames per second and photodiode traces. Based on these measurements, an empiric definition of the power threshold of mode instabilities is introduced. Additionally, it is shown that the temporal dynamics show a transition zone between the stable and the unstable regimes where well-defined periodic temporal fluctuations on ms-timescale can be observed. Finally, it is experimentally shown that the larger the mode-field area, the slower the mode-instability fluctuation is. The observations support the thermal origin of mode instabilities.

Physical origin of mode instabilities in high-power fiber laser systems

Mode instabilities, i.e. the rapid fluctuations of the output beam of an optical fiber that occur after a certain output power threshold is reached, have quickly become one of the most limiting effects for the further power scaling of fiber laser systems. Even though much work has been done over the last year, the exact origin of the temporal dynamics of this phenomenon is not fully understood yet. In this paper we show that the origin of mode instabilities can be explained by taking into account the interplay between the temporal evolution of the three-dimensional temperature profile inside of the active fiber and the related waveguide changes that it produces via the thermo-optical effect. In particular it is proposed that non-adiabatic waveguide changes play an important role in allowing energy transfer from the fundamental mode into the higher order mode. As it is discussed in the paper, this description of mode instabilities can explain many of the experimental observations reported to date.

26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality

We demonstrate a Q-switched fiber laser system emitting sub-60 ns pulses with 26 mJ pulse energy and near-diffraction-limited beam quality (M2<1.3). In combination with a repetition rate of 5 kHz, a corresponding average output power of 130 W is achieved. This record performance is enabled by a large-pitch fiber with a core diameter of 135 µm. This fiber allows for effective single-mode operation with mode field diameters larger than 90 µm even at average output powers exceeding 100 W.

Thermally induced waveguide changes in active fibers

Thermally induced waveguide changes become significant for very large mode area fibers. This results in a reduction of the mode-field diameter, but simultaneously in an improvement of the beam quality. In this work the first systematic experimental characterization of the reduction of the mode-field diameter in various fibers during high-power operation is carried out. It is shown that the reduction of the mode-field diameter shows a characteristic behavior that scales with the core size but that is independent of the particular fiber design. Furthermore, the strength of the actual index change is experimentally estimated, and its use to overcome avoided crossings is discussed and experimentally demonstrated.

High-speed modal decomposition of mode instabilities in high-power fiber lasers

A high-speed mode analysis technique is required to gain fundamental understanding of mode instabilities in high-power fiber laser systems. In this work a technique, purely based on the intensity profile of the beam, is demonstrated to be ideally suited to analyze fiber laser dynamics. This technique, together with a high-speed camera, has been applied to the study of the temporal dynamics of mode instabilities at high average powers with up to 20,000 frames per second. These measurements confirm that energy transfer between the fluctuating transversal modes takes place in millisecond-time-scale.

The influence of index-depressions in core-pumped Yb-doped large pitch fibers

Rare-earth doped photonic crystal fibers rely ideally on an index matching of the doped core to the surrounding glass to work properly. Obtaining a perfect index matching is technologically very challenging, and fiber manufacturers opt for targeting an index depression instead, which still ensures the influence of the photonic structure on the light propagation. In this paper the analysis of the influence of this core index depression on the higher-order mode discrimination and on the beam quality of the fundamental mode of different designs of core-pumped active large pitch photonic crystal fibers is discussed. The most promising design is evaluated in terms of mode area scaling with a view to mode field diameters above 100 µm. Detailed requirements on the accuracy of the core index matching are deduced.

2.4 mJ, 33 W Q-switched Tm-doped fiber laser with near diffraction-limited beam quality

We report on a high pulse energy and high average power Q-switched Tm-doped fiber oscillator. The oscillator produces 2.4 mJ pulses with 33 W average power (at a repetition rate of 13.9 kHz) and nearly diffraction-limited beam quality. This record performance is enabled by a Tm-doped large-pitch fiber, which allows for large core diameters in combination with effective single-mode operation.